But where to get such gratis grub on a regular basis? It is one thing to munch your way around a food festival, methodically trying every free sample on site. It is even better if, as a trusted regular, your local restaurant starts road-testing its dishes on you. The holy grail, however, is finding someone who is willing to regularly pick up your restaurant bill. Which is why life as an undercover "mystery diner" sounds so appealing.

Whether individual restaurants or third-party consultancies, there are a surprising number of people out there willing to give you free food and drink (and occasionally even a fee) in return for your feedback on their food and service. Leicester's Entropy runs a scheme which rewards selected "mystery guests" with vouchers in return for their answering a detailed questionnaire (about the volume of background music, and whether or not branded beer glasses were served badge-facing, etc). Similarly, Pizza Express club members, can claim a £10 voucher each quarter after sharing their feedback online.

It is a job which the best mystery diners take pretty seriously, too. As the rise of the food blog has illustrated, there are a lot of enthusiastic people out there, keen to share their opinions about how restaurants are run. Such people are natural mystery diners.

"Motivations vary," says Steven Pike, MD of the Mystery Dining Company (MDC), whose clients include Ask, Zizzi, Wagamama, Prezzo and Le Bistrot Pierre. "It enables people to go out and have that paid for. It's a way of extending your lifestyle. But it does attract a certain type of person, one who has opinions, wants to make a difference and who is happy to commit their experiences to an online form. Which is important for us. Those people who just think it's a free meal, and that's it - the people who don't care - won't get very far."

MDC currently has 2,000 assessors on its books. Some do a couple of reports each year, others over 10 a month. Every assessor completes a lifestyle profile when they first sign up so they can be offered suitable opportunities, depending on the client restaurant's requirements (they might want visits from families, retirees, young professionals etc). Assessors are free to pick and choose from a regularly updated list of suitable tasks in their area, the idea being that visits should fit into their schedule. Inducements and / or travel expenses may be offered, usually if MDC is looking for reports from a remote location, but generally no payment is made. "If people are doing it to earn money," argues Pike, "you start to skew the responses. They're doing it for the wrong reasons." All food and drink expenses are covered, however, to pre-agreed limits.

Visits are sometimes scripted (for instance, assessors might have to test the staff's product knowledge or the complaints procedure) and the subsequent reports take about an hour to complete. These can take several forms, from a detailed multiple-choice questionnaire (typical of a large chain which is testing its standardisation procedures) to more subjective questions that might require several paragraphs in answer. Often staff bonuses and incentive schemes - not to mention occasional disciplinary proceedings - are closely tied into the data produced by mystery diners for the client restaurants.

That might all sound like reasonably hard work, an onerous responsibility even, but MDC, which rarely advertises, and then only in online forums, already rejects around half the people who apply and is massively oversubscribed in certain areas, especially in the south east.

Assessors, says Pike, have to be observant, comfortable in restaurant environments, willing to absorb constructive feedback on their own reports and have good written English. But they must also work to the brief: "We're not looking for amateur critics who can write flowery prose. We're looking for people who can express basic facts and perceptions clearly."

That requirement explains MDC's strict rule that you can't get drunk on the job. Assessors are allowed two glasses of wine. Drink more and the report will be rejected and you won't be reimbursed. On rare occasions, determined drinkers have opened a separate tab for their booze, only to be found out when, after submitting their receipt, the client restaurant has checked that bill with the spend at that specific table, on the night of the assessor's visit. Which must be pretty humiliating.

Parsimonious alcohol intake aside, does life as a mystery diner appeal? Have you worked as one? Did it involve doing things which made you feel uncomfortable? Or is there a better way of dining for free?